DESCRIPTION: Human granulocytic ehrlichiosis (HGE) is an emerging tick-borne acute febrile illness. The agent of HGE is closely related to or identical to the horse pathogen Ehrlichia equi and is transmitted by Ixodes scapularis, the same tick that transmits the agents of Lyme disease and babesiosis. The HGE agent infects blood polymorphonuclear neutrophils resulting in neutropenia and a thrombocytopenia. HGE may be a mild to potentially fatal disease. HGE was first described in Minnesota and Wisconsin in 1994 and subsequently more than 400 cases have been described in the U.S. In contrast to Lyme disease which frequently occurs in children, the rates and severity of HGE increase with age. This age-specific incidence of HGE and our large aging population presents a potentially major public health problem with corresponding economic consequences. This emerging public health problem highlights the importance of early diagnosis and treatment in preventing complications of infection, and targeting of prevention and control studies to populations at highest risk. The research proposed in this application is designed to aid in achieving the above goals by analyzing the epizootiology of HGE in the North Central U.S. Our specific aims are: to investigate the hypothesis that white-tailed deer serve as a large mammal reservoir of the agent of HGE and to identify the small mammal reservoirs of the HGE agent.